1. Field of the Invention
The present invention relates to a system for locating short circuits and open circuits with respect to a test device inserted between first and second sections in a two-lead line. The other end of the first line section is selectively connected to means for generating a predetermined voltage and to means for detecting a first predetermined frequency. The test device comprises means series-connected on one of the line leads for detecting the in-line current, means across the line leads for detecting the predetermined voltage and means for generating a signal having the first frequency that modulates the in-line current.
Although the invention is not restricted to this type of embodiment, reference will be made hereinafter to the location of faults in a telephone subscriber two-lead line that links a telephone exchange to a telephone subscriber installation. The faults located by the test system fall into two categories. The first category covers short circuits, loops, insulation defects or low resistance shunts between the two line leads and between one of the line leads and the ground. The second category covers open circuits, breakdowns, cut-offs or high resistances in at least one of the line leads.
2. Description of the Prior Art
A test system as defined above is described in U.S. patent application Ser. No. 339,047 filed on Jan. 13, 1982, now U.S. Pat. No. 4,459,437 and entitled "Test System For Locating Breaks and Short Circuits in a Two-lead Line ", now U.S. Pat. No. 4,459,437, dated July 10, 1984.
In this test system, the test device is sensitive to a predetermined voltage level and a predetermined current level. The response upon applying this voltage, respectively this current, is a frequency signal F.sub.1, respectively F.sub.2, or perhaps F.sub.3 when the device comprises second current detecting means on the other line lead. The voltage and current thresholds in this system are high; for instance, the voltage threshold lies at 230 volts d.c. Metal continuity is always required and activation of the frequency generating means in the test device calls for a high voltage which can cause deterioration in teleprocessing equipments and analogous means at the end of a second line section on a subscriber installation side.
Other devices based on voltage threshold detection have been marketed in the United States.
The CIDCOMM (remote line disconnector) test device is activated by a 130 volt d.c. pulse on a line lead with respect to ground. It gives rise to a short circuit for 20 seconds and then an open circuit for the next 20 seconds and reverts to the rest position.
The TII (station disconnect) test device is activated by a 130 volt d.c. voltage pulse on a line lead with respect to ground. It provides an infinite impedance for 25 seconds between any one of the line leads and ground together with an impedance between the line leads of either infinity or amounting to 100 k.OMEGA., depending on the supply bias of the line lead; then it reverts to rest.
The SEISCOR (loop circuit verifier) test device detects an inversion in bias of the 0/48 volt battery voltage at the exchange and in response to this invention emits a 900 Hz tone until such time as the supply bias reverts to the initial state.
The MELCO (remote disconnect device) test device is activated by a 130 volt d.c. voltage pulse on a line lead with respect to ground, and provides a 47 .mu.F capacity in series with a 10 k.OMEGA. resistance for 15 seconds between the two line leads; then it reverts to rest.
All these test devices have the drawback of requiring metal continuity and a high activation voltage. Furthermore, they need either a "good" ground connection at the subscriber installation end, usually less than 100 ohms, or line lead marking, or both these constraints simultaneously.